In an electrochemical cell, the electrochemical reactions at the involved electrodes are decisively influenced by the half-cell overvoltages. These overvoltages include both contributions from the activation and from transport losses of the corresponding half cells. It is therefore considered to be a primary objective to minimize these contributions within the scope of a novel cell concept. One of the most common previous methods for measuring half-cell overvoltages is a method using a reference electrode (RE).
A reference electrode is always aimed at measuring the potential Φ of the electrolyte membrane at an arbitrary point between two electrodes. Having knowledge of this potential makes it possible, in principle, to separate the cathode overvoltage and the anode overvoltage. However, it has been found that suitable positioning of the reference electrode is not readily apparent.
A typical option for positioning a reference electrode within an electrochemical cell is to dispose the reference electrode on one cell side, at a certain distance Lgap from the edges of the working electrode, which are aligned with one another. For this purpose, a design according to FIG. 1 is proposed, in which the two electrodes or the lateral edges thereof are aligned with one another.
Within the scope of simulations, B. Adler et al., in J. Electrochem. Soc. 149, E166 (2002), have shown that the distance Lgap must be greater than three times the membrane layer thickness lm. At this distance, the inhomogeneities that are regularly caused by the edge of a working electrode, and the losses of potential resulting from the hydrogen oxidation reaction (HOR), are negligible, and the potential measured at the reference electrode regularly corresponds to the membrane potential at any arbitrary point along the z-axis between the anode and the cathode in the working area.
The measurements of the membrane potential Φ and of the electrode potentials then allow the overvoltages at the anode and the cathode to be separated.
Furthermore, with the aid of reference electrodes it is possible to carry out electrochemical impedance spectroscopy between the reference electrode and any other cell electrode. This technique has already been applied in the field of solid oxide fuel cells (SOFC).
In electrochemical cells, in which protons act as charge carriers, the potential of a hydrogen-fed reference electrode generally corresponds to the membrane potential Φ at the location of the reference electrode, neglecting the voltage losses due to the hydrogen oxidation/generation reaction.
In the implementation for positioning a reference electrode in a fuel cell according to FIG. 1, however, two essential problems occur. Unless indicated otherwise, it is assumed hereafter that the anode is grounded, and that all potentials are measured with respect to the anode.
Initially, it shall be noted that even a very small shift (δ) in the alignment of the edges of the working electrodes with respect to the counter electrode nonetheless causes a large change in the potential at the reference electrode. This effect has already been sufficiently discussed in the literature on solid oxide fuel cells (SOFCs). Several suggestions on minimizing this effect have also already been described there. An analogous application of this solution to PEMFCs has likewise already been proposed in the literature, which reports on a system of working electrodes precisely aligned by way of laser ablation.
A further problem, which occurs even in electrochemical cells in which the edges of the working electrodes are precisely aligned (δ=0), is that, even though the value of the potential Φ measured by the reference electrode corresponds to a point on the z-axis between the working electrodes, the exact position of this point on the z-axis is not known. It is therefore not possible, in such a case, to unambiguously evaluate and interpret the measured DC voltage signal of the reference electrode. In this case as well, the separation of the half-cell overvoltages can only take place by way of impedance spectroscopy.
The latest developments by A. A. Kulikovsky and P. Berg (DE 102015001572.9), however, demonstrate that it is possible to determine the overvoltage of a working electrode in a fuel cell by measuring the potential of a reference electrode compared to the grounded counter electrode. A fuel cell according to FIG. 2 is used for this purpose. This includes a large-surface-area (endless) working electrode, which in this instance is a cathode, and a counter electrode, which has at least one lateral edge. Within the scope of the invention, an electrode is referred to as having a large surface area (being endless) when the extension thereof corresponds at least to 10 times the parameter l*, and in particular when the extension of the electrode is greater than the membrane layer thickness by several orders of magnitude. As a result of the edge of the counter electrode, an electrode-free region is obtained, compared to the working electrode, on the electrolyte membrane surface adjoining the counter electrode, in which the reference electrode is disposed on the electrolyte membrane surface. Due to the fact that the working electrode no longer has an edge in the vicinity of the working area, there is likewise no longer a problem of the alignment of the electrode edges.
It was possible to demonstrate that the minimum distance Lgap between the reference electrode and the edge of the counter electrode must meet the following condition:
                    L        gap            ≥              3        ·                  l          *                      =          3      ·                                                  σ              m                        ⁢                          b              ox                        ⁢                          l              m                                            2            ⁢                          j              ox              0                                            ,where σm=ionic conductivity of the electrolyte membrane (Ω−1 cm−1), box=Tafel slope of the half cell for the electrochemical reaction of the cathode, lm=membrane layer thickness (cm) and jox0=exchange current density of the catalyst of the cathode per unit of electrode surface in (A cm−2). At distances between the reference electrode and the counter electrode which correspond at least to this minimum distance, it can be ensured that the measurement of the potential of the reference electrode substantially corresponds to the overvoltage of the working electrode, which is to say the cathode.
Using typical cell parameters (see Table 1), this yields a minimum distance Lgap on an order of magnitude of several centimeters, in the case of the reference electrode being disposed on the anode side for ascertainment of the overvoltage on the cathode side. This distance, however, is a very large value for a test fuel cell of normally 10 cm*10 cm or smaller.